1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and an apparatus for transmitting a downlink reference signal in a wireless communication system supporting multiple antennas.
2. Discussion of the Related Art
A Multiple Input Multiple Output (MIMO) system refers to a system for improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas. MIMO technology includes a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme is suitable for data transmission of a user equipment (UE) which moves at a high speed, because transmission reliability is increased or a cell radius is increased through diversity gain. The spatial multiplexing scheme can increase data transfer rate without increasing system bandwidth by simultaneously transmitting different data.
In a MIMO system, each transmission antenna has an independent data channel. The transmission antenna may be a virtual antenna or a physical antenna. A receiver estimates a channel with respect to each transmission antenna and receives data transmitted from each transmission antenna. Channel estimation refers to a process of compensating for signal distortion due to fading so as to restore the received signal. Fading refers to a phenomenon in which the intensity of a signal is rapidly changed due to multi-path and time delay in a wireless communication system environment. For channel estimation, a reference signal known to both a transmitter and a receiver is necessary. The reference signal may be abbreviated to RS or referred to as a pilot signal according to the standard.
A downlink reference signal is a pilot signal for coherent demodulation, such as a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH). The downlink reference signal includes a Common Reference Signal (CRS) shared among all UEs in a cell and a Dedicated Reference Signal (DRS) for a specific UE. The CRS may be referred to as a cell-specific reference signal. The DRS may be referred to as a UE-specific reference signal or a Demodulation Reference Signal (DMRS).
In a system having an antenna configuration (e.g., a system according to the LTE-A standard supporting eight transmission antennas) developed as an extension of a legacy communication system (e.g., a system based on LTE release 8 or 9 standard) supporting four transmission antennas, DMRS-based data demodulation has been considered in order to support efficient reference signal management and develop a transmission scheme. That is, in order to support data transmission through extended antennas, DMRSs for two or more layers may be defined. Since the DMRSs are precoded using the same precoder as data, it is possible to easily estimate channel information for demodulating data at a reception side without separate precoding information.
A downlink reception side may acquire precoded channel information with respect to the extended antenna configuration through DMRSs. However, in order to acquire non-precoded channel information, separate reference signals are required in addition to the DMRSs. In a system based on the LTE-A standard, reference signals for acquiring Channel State Information (CSI) at a reception side, that is, CSI-RSs, may be defined.